[CAD_CAM_EDM_DRO] SLA7062 Sync Mode (was New Article bla bla)

This is very interesting and confirms what I was just about to write.

The other Dave's analysis assumes that stepping errors will be cumulative
when microstepping with the SLA7062 and switching to sync mode when paused.
I suspect that assumption is incorrect, for the following reasons. For one
thing, the SLA7062 data sheet does not say WHY it is not recommended. I
would bet the reason they say that is because they must assume that if you
are microstepping you need the extra position accuracy when holding. This
would be the worst case scenario, and semiconductor manufacturers ALWAYS
assume the worst case scenario in their specs. (I worked in the
semiconductor biz for many years). It seems very unlikely to me that
entering sync mode would get the step logic out of phase. More likely is
that the PWM current regulator is not accurate enough to hold the motor in a
position beyond a half step precision. Keeping track of steps is a logic
function, and is (or at least should be) unaffected by the PWM operation.

An easy way to put this question to rest would be to setup a mill with a
dial gage against a block, zero it, and generate series of moves and stops
in one direction with random distances, and then the sum of those moves at
once (taking into account backlash) in the other. If the dial gage returns
to zero within the tolerance of your mechanics, then the driver is not
missing steps. Dave's experience below tends to support this outcome. Maybe
I'll do this test myself when I have time, though I am pretty busy right now
filling orders for the drivers I sell. (I could have worse problems.)
--
Phil Mattison
http://www.ohmikron.com/

----- Original Message -----
From: Codesuidae <codesuidae@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Monday, May 08, 2006 9:07 AM
Subject: Re: [CAD_CAM_EDM_DRO] Re: New Article Posted


> Phil Mattison wrote:
> > > So anyone using these boards will need to turn OFF microstepping, OR
the
> > > auto current and noise reduction. Can't use both without losing
steps...
> >
> > That is correct, assuming you consider half-stepping not microstepping.
Step
> > divisions greater than 2 (the default) are not recommended for the 7062
in
> > sync mode. For most applications using the PC parallel port, the
required
> > pulse rate would be too high anyway if you used higher microstepping
rates.
> >
>
> Do you have some more information about this? I was running 8x
> microstepping from my computer parallel port on a HCNC controller and
> never had any issues. Going up to the next level (16?) gave some
> problems when jogging, it seemed like that was beyond the rate that the
> parallel port could handle. There was some delay (0.25-0.5s) between
> releasing the jog keys and the axis stopping.
>
> There was a definite advantage to 8x microstepping over 4x or
> half-stepping on my Taig mill. I was able to run acceleration at
> 200,000 Hz as opposed to less than 1/10th that with 4x.
>
> > I don't see where there is all that much to be gained anyway by high
> > microstep rates in this case. The extra efficiency is trivial.
>
> In my case there was a huge performance difference. Naturally it
> depends on the characteristics of the machine, but I'd not like not
> having the choice.
>
> I'm curious about the problem with sync mode and lose of steps in
> microstepping mode. Is this as a result of the reduced holding torque
> at non- full or half step positions? I always wondered how it was that
> the computer could run with the controller in microstep mode without
> knowing where the full and half step positions where. It always seemed
> to me that it would be highly likely to stop in positions the stepper
> couldn't hold, possibly resulting in lost steps.
>
> Dave K
>
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